In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas via an arc discharge. One class of HID lamps comprises inductively coupled electrodeless lamps which develop and maintain an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In such a lamp, the high pressure fill within an arc tube is initially broken down by an electric discharge, and the resulting discharge plasma is excited by radio frequency (RF) current in an excitation coil surrounding the arc tube. The arc tube and excitation coil assembly act essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary coil inductively coupled to the primary coil. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which substantially closes upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the plasma within the arc tube.
The toroidal discharge in an inductively coupled HID arc tube is generally more difficult to start than the discharge in a conventional arc tube having electrodes serving as terminals for the discharge. There are several reasons for this. First, the absence of electrodes eliminates the beneficial role which electrodes often play in starting electroded arc tubes. For example, without the electrodes, there is no opportunity for electric field concentrations at the electrode tip and no opportunity for generating initial electrons by physical processes at the surface of the cathode electrode such as by thermionic emission, field emmission, or ion bombardment. Second, it is very difficult to inductively generate the very high electric fields required for breakdown of the relatively high-pressure fill gas within the arc tube. Third, we utilize as the buffer gas in our arc-tube fill a high pressure inert gas, rather than mercury. For example, in one embodiment of our invention, we utilize as the buffer gas within our arc-tube fill krypton or xenon having a room-temperature pressure of 250 torr or more. This inert-gas pressure is approximately ten times higher than the inert-gas pressure which is desirable for initial starting breakdown.
There have been a number of approaches tried or suggested for initiating the arc discharge in the high pressure inert gas arc-tube fill of an electrodeless lamp. One early approach involves lowering the gas pressure of the fill, for example, by first immersing the arc tube in liquid nitrogen so that the gas temperature is decreased to a very low value and then allowing the gas temperature to increase. As the temperature rises, an optimum gas density is momentarily reached for ionization, or breakdown, of the fill to occur so that an arc discharge is initiated. However, the liquid nitrogen method of widespread commercial use.
More recent approaches have involved the use of a variety of metallic "starting aids", which typically serve to increase the electric field for starting. These metallic starting aids are usually located outside the arc-tube envelope but in some cases have been starting electrodes which enter the arc-tube envelope through seals. Examples of such metallic starting aids are shown in U.S. Pat. Nos. 4,894,589-Borowiec, 4,894,590-Witting, 4,902,937-Witting, and in applications Ser. No. 417,404-Witting filed Oct. 5, 1989, Ser. No. 527,500-ElHamamsy et al filed May 23, 1990, Ser. No. 527,502-El-Hamamsy et al, filed May 23, 1990, Ser. No. 07/527,502 Roberts et al, filed May 23, 1990, all of which are assigned to the assignee of the present invention and are incorporated by reference in the present application.
There are some disadvantages in using a metallic starting aid. For example, if the metallic starting aid is of such a character that it remains in place during lamp operation, it may serve as a vehicle for a life-limiting mechanism such as sodium loss, degradation of the arc-tube envelope wall, or seal failure. On the other hand, if a metallic starting aid is of such a character that it is removed or withdrawn after starting, then the complications and expense involved in controlling such moving part are introduced into the lamp design. Furthermore, a movable starting aid tends to change the impedance matching requirements of the energizing circuit for the excitation coil.